1. Field of the Invention
The present invention relates to an image forming apparatus and, more particularly, to an image forming apparatus having a recording head which discharges liquid droplets.
2. Description of the Related Art
An ink-jet recording apparatus is used as an image forming apparatus incorporated in a printer, a facsimile machine, a copy machine, a plotter or a multi-function peripheral (MFP). The inkjet recording apparatus is known as an image forming apparatus of a liquid discharge recording system using a recording head which can discharge ink droplets. The image forming apparatus of a liquid discharge recording system performs image-formation by discharging ink droplets from a recording head toward a recording paper while the recording paper is moved. Here, the term “image-formation” includes meanings of recording, letter-printing, photo-printing and printing. There are two types of image forming apparatus having a recording head to discharge ink droplets, one is a serial-type image forming apparatus and the other is a line-type image forming apparatus. The serial-type image forming apparatus forms an image by a recording head discharging liquid droplets while the recording head is moved in a main-scanning direction. The line-type image forming apparatus forms an image by discharging liquid droplets from a recording head in a state where the recording head is not moved.
In this specification, the term “image forming apparatus” means an apparatus to perform an image-formation by causing ink droplets to land on a medium such as paper, string, fiber, cloth, leather, metal, plastic, glass, wood, ceramics, etc. The term “image-formation” means not only producing an image having characters or graphics onto a medium but also producing an image having no meaning such as a pattern. The term “ink” is not limited to a general meaning of ink, and is used as an all-inclusive term for liquid which can form an image, such as a recording liquid, a fixation processing liquid, a liquid, and a liquid resin. The term “recording paper” is not limited to a general meaning of paper, and includes an OHP sheet and a cloth. The term “recording paper” is used as an all-inclusive term which encompasses a medium to be recorded, a recording medium, a recording paper, a paper for recording, etc. The term “image” is not limited to a planar matter, and includes an image given to a stereoscopically-formed matter or an image formed in a three-dimensional shape.
There is known an image forming apparatus, which creates a plurality of drive pulses (discharge pulses) to discharge liquid droplets within one drive cycle in a time-series manner and outputs the drive pulses as a common drive waveform; when creating relatively large dot, two or more pulses are selected to discharge a plurality of liquid droplets and cause the plurality of liquid droplets to be combined in one while the liquid droplets are flying, which results in creation of a dot having a size of a plurality of liquid droplets; and a non-discharge pulse, which drives a head without discharging liquid droplets, is included in the common drive waveform in order to discharge liquid droplets stably by selecting the non-discharge pulse to perform a fine drive.
For example, Japanese Laid-Open Patent Application No. 2001-315332 (Patent Document) discloses a drive method of an ink-jet printer, which comprises a plurality of nozzles for discharging ink droplets, and a pressure generating means provided to each nozzle for applying a pressure to ink in each nozzle, wherein printing is performed while moving a recording paper relative to the nozzles. According to this drive method, a first voltage pulse and a second voltage pulse are applied to the pressure generating means given an instruction to discharge ink droplets in synchronization with a reference signal, the first voltage pulse having amplitude by which ink droplets can be discharged, and the second voltage pulse causing ink inside the nozzle to flow within the nozzle. The second voltage pulse is applied to the pressure generating means corresponding to the nozzle which is not provided with an instruction of discharging ink droplets (the nozzle of which passed time or passed reference signal number from ink discharge of last time is equal to or greater than a threshold value) in order to attempt a reduction in power consumption.
However, the structure disclosed in the above-mentioned Patent Document does not sufficiently reduce power consumption. For example, a typical text (character) document has a printed area, which is 5% to 10% of the entire area of the document and the rest of the area is blank in many cases. When printing such an image by an inkjet recording apparatus having a plurality of nozzles, there are nozzles that do not discharge liquid droplets at all.
Therefore, according to the structure disclosed in the above-mentioned Patent Document, where a minute drive pulse is applied in a condition depending on only a discharge state of a nozzle before a current drive cycle (reference signal), that is, a condition where droplet discharge is not performed during a predetermined time period or a predetermined drive cycle, even if the number of times may be reduced in the above-mentioned case, a minute drive pulse is still applied, which results in a wasteful consumption of electric power.